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1
Soak both black and yellow mustard seeds in cold water for 30 minutes, then drain — the soak softens the seed coat so the paste grinds to a truly smooth, bitter-free result rather than a gritty, broken one. Transfer to a stone grinder or blender with 2 green chillies, the turmeric, a pinch of salt, and enough water to facilitate grinding. Grind to a smooth, cohesive paste — no graininess should be detectable when a little is rubbed between finger and thumb.
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2
Thin the paste with the remaining water to a pourable, sauce-like consistency. Season conservatively with salt, bearing in mind that the mustard's sharpness amplifies salt perception throughout cooking.
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3
Heat the mustard oil in a flat-bottomed bronze kadai or iron tawa over a medium flame until it just begins to smoke, so the raw, acrid edge of the oil is driven off and what remains is the clean, warm pungency that the dish needs. Reduce the heat to low before proceeding.
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4
This is where the dish lives or dies: pour the mustard paste into the pan, nestle the hilsa steaks in a single layer, and cover tightly. The sauce must not boil vigorously at any point — a vigorous boil drives off the volatile compounds that carry the dish's defining sharp, sulphurous pungency — so maintain a bare, covered simmer throughout.
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5
Cook, covered, until a thin film of oil separates and pools at the surface of the sauce, which tells you the fish is cooked through and the sauce has come together correctly. The flesh of the hilsa should be just set and yield without resistance when pressed gently at its thickest point.
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6
Transfer the steaks carefully to a serving dish with the sauce spooned over. Finish with a drizzle of raw mustard oil, and serve immediately over plain gobindobhog rice, with sliced raw green chilli and raw onion alongside.